(1) Genes are long sequences of (coding) triplet-bases. Are those triplet-bases (a) always contiguous (allowing for non-coding introns interspersed along the sequence)? Is there therefore always a point on the DNA strand that is the start of the gene, and another that is the end (allowing for promoters etc etc), and all the relevant exons for the gene are between that start point and end point?

Or (b), can genes be 'assembled' from fragments of exons+introns that are non-contiguous on a single strand, but which are on separated portions of the strand, possibly even scattered around different chromosomes? If so, are those contributory segments transcribed into a single, contiguous piece of (immature) messanger RNA, or are they transcribed into multiple mRNAs, which are then all processed into mature RNA, introns spliced out etc, to carry the contiguous sense of the gene ready for translation into the corresponding polypeptide chain?

(2) Are genes 'unique' - ie, are the triplet-bases that constitute any one gene ever used in any other gene? ie, can genes 'overlap' along a single contiguous sequence (if (a) above) or be differently assembled (if (b) above) (I appreciate that gene families have arisen from multiple replication of 'ur-genes' way back when!)

(Are there ever multiple copies of the same gene?, ie, as opposed to a single gene being transcribed multiple times at periods of upregulation of gene-expression, resulting in multiple copies of mRNA, each slightly 'lagging' behind the one in front on the transcription 'assembly line')

If exons can contribute to more than one gene, is it still the case that only ONE of those genes is ever expressed - ie, that whether a particular exon contributes to a particular gene instead of another depends upon silencing/differentiaton? Or can a differentiated cell

So, in brief - are genes always contiguous (allowing for introns etc) along a single strand of DNA, and are the exons in any gene ever contributory to more than one gene?

I'm under the impression that the first one of your options is the correct one. I do not know of any genes that are constituted from exons from several different other ones. However a section of DNA can be spliced in any number of different ways, resulting in several different proteins and in this way codons can be used for different proteins - after all each of them codes just for amino acids and 20 of them make every protein in the body!

Splicing also varies between cells - so some might want a AMPA receptor for glutamate and another a NMDA receptor - and so there will be differential splicing of the same gene in each of these cells. So the exons are being used for different codes in different cells.

Hope this helps - please point out if I've missed answering something or made it unclear!

So, is this right - a particular sequence of contiguous coding triplet-bases (allowing for interspersed introns) that constitutes one 'end-to-end' gene (and therefore one 'end-to-end' polypeptide chain), can also be 'reassembled' in a different sequence of coding triplet bases to yield a different polypeptide sequence, but this 'reassembly' is always within the 'end to end' gene sequence? ie, one gene can, depending on how its constituent coding-triplets are sequenced, be expressed as more than one protein?

On your second paragraph, my knowledge of splicing is (so far!) trailing (considerably!) but am I right in thinking that what you've said is because RNA splicing can accomplish base substitution (RNA editing) (eg, AGA/arg in the DNA is base-changed to GGA/Gly in the mRNA, thus changing the properties of the AMPA receptor that that AGA/GGA contributes to )? (I've just looked that up in my genetics texbook!) -

The textbook helpfully also says:

"The observation of multiple RNAs for the AMPA receptor raises the possiblity that RNA editing may fine-tune the regulation of neuronal signaling by allowing cells to synthesize - from the same DNA blueprint - proteins with different properties, at different times in development and different stages of neuronal activity."

Which goesto show I should have checked first before posting this question!

there can be several copies of some genes. E.g. in maize whole parts of chromozomes were duplicated not that long time ago and thus they could be considered as copies of the same gene (although there has been some mutations already). But surely there are several copies of genes for tRNA, which must be present in large amounts in cells.